bhyve: build SPCR ACPI table

OVMF ships some static ACPI tables. This worked in the past but won't
work in the future when we support devices like tpms. They require a TPM
ACPI table. So, we have to dynamically create ACPI tables depending on
the bhyve configuration.

Bhyve has much more information about the system than OVMF. Therefore,
it's easier for bhyve to build up some ACPI tables. For that reason, it
would be much better to use the ACPI tables provided by bhyve instead of
building some tables by OVMF.

At the moment, OVMF always creates a SPCR table. Maybe someone depends
on it. So, we have to build it by bhyve too before we can patch OVMF to
install the tables provided by bhyve.

Reviewed by: markj
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D37591